There are some oil-filled electrical apparatuses such as an oil-filled transformer which may include an insulating oil containing a sulfur component. In this case, it is known that the sulfur component reacts with a copper part in the oil to produce copper sulfide exhibiting electrical conductivity, which leads to a problem that dielectric breakdown occurs. However, the details of the mechanism for producing copper sulfide are not known. Furthermore, it is difficult to diagnose a failure of the oil-filled electrical apparatus resulting from production of copper sulfide, without suspending the operation of the existing apparatus. Thus, the techniques for selecting an insulating oil suppressing sulfidation corrosion and for removing sulfur compounds are employed to reduce its tendency to produce copper sulfide.
For example, Japanese Patent Document 1 described below discloses a method for selecting an insulating oil which suppresses sulfidation corrosion. A certain amount of insulating oil and a copper plate having a predetermined surface area are enclosed in a container and heated at a predetermined temperature for a predetermined period of time. Then, the content percentages of the dissolved copper in the oil and sulfate ions contained in the insulating oil are measured to obtain the sum of the percentages, which is used to diagnose the sulfidation corrosiveness of the insulating oil.
Furthermore, a method for diagnosing a failure of an oil-filled electrical apparatus by detecting a certain type of substance is known. For example, Japanese Patent Document 2 described below discloses a method for diagnosing failures such as overheating or discharging within the apparatus based on whether or not there is detection of acetic acid [CH3COOH], 3-pentanone [CH3CH2COCH2CH3], 2.5-dimethylfuran [C6H8O], butyraldehyde [CH3CHCHCHO], 2-methoxyethanol [C3H8O2], methanethiol [CH3SH], dimethyl sulfide [(CH3)S2], ammonia [NH3], 1.3-diazine [C4H4N2], methyl vinylacetylene [C5H6], and 2-methyl-1.3-butadiene [C5H8] which are not detected from the oil-filled electrical apparatus during normal operation, but likely to be detected only when overheating or discharging occurs.
Furthermore, Japanese Patent Document 3 described below discloses a diagnostic method using the amount of each gas such as carbon dioxide [CO2], carbon monoxide [CO], methane [CH4], hydrogen [H2], ethane [C2H6], ethylene [C2H4], and acetylene [C2H2] contained in the oil as an index showing the conditions including deterioration and failures of an oil-filled transformer, a reactor, an oil-filled electrical apparatus such as an automatic voltage regulator, an oil-immersed cable, and the like.
Furthermore, Japanese Patent Documents 4 and 5 described below each disclose a diagnostic method for an oil-filled transformer in which hydroxymethylfurfural and furfural are detected and used as an index of deterioration of the insulating paper.
Furthermore, Japanese Patent Document 6 described below discloses a method for detecting a sulfur compound and a product thereof in the oil in order to diagnose flow electrification of an oil-filled electrical apparatus.    Patent Document 1: Japanese Patent Laying-Open No. 07-335446    Patent Document 2: Japanese Patent Laying-Open No. 09-072892    Patent Document 3: Japanese Patent Laying-Open No. 2000-241401    Patent Document 4: Japanese Patent Laying-Open No. 05-315147    Patent Document 5: Japanese Patent Laying-Open No. 08-124751    Patent Document 6: Japanese Patent Laying-Open No. 2005-223104